The two Boeing aircrafts that crashed in Indonesia and Ethiopia both lacked a safety feature the manufacturer was selling as optional.

As more information comes about on the 737 MAX 8 jets and aircrafts around the world stay grounded, airlines are checking their fleets to see if they’re equipped with the safety options.

This could have helped pilots identify drastic altitude changes and speed irregularities before the crashes. Lion Air and Ethiopian Airlines reportedly opted out of purchasing these features, according to The New York Times.

The angle of attack sensor (or AOA) measures the angle of the jet’s nose as well as air passing over the wings. This information tells the MCAS, or Maneuvering Characteristics Augmentation System, to push the nose back down if the aircraft was climbing too quickly or steeply, which could cause the aircraft to stall.

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The add-on is a light that displays this information in the cockpit, while another light tells pilots when the measurements of the sensors aren’t corresponding.

Canada’s largest air carrier, Air Canada, says they purchased both safety features on all 24 of its MAX planes, while a WestJet spokesperson said their 13 aircrafts have only one sensor installed—the disagree light.

Boeing announced it will make the light warning the sensors don’t match mandatory on its MAX aircrafts, while a software update will be rolled out shortly. Until then, all 737 MAX 8 plans remain grounded.

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Winston is currently a freelance technology and travel broadcast journalist, consultant, and is the creator and founder of Master Travellr—Canada’s destination for travel news, guides, and budget recommendations.